Coaxial antenna



States 8 Claims. (Cl. 343-779) The present invention relates generally to microwave antennas and more particularly to 'a coaxial antenna.

areas The primary object of this invention is to provide a coaxial antenna having a first reflector hereinafter referred to simply as a transmitter reflector mounted concentrically in front of a second reflector hereinafter referred to simply as a receiver reflector, the receiver feed comprising a third reflector and a feed unit, the latter being mounted at the center of the larger receiver reflector and the received signal being directed thereto from said third reflector which is a plane reflector mounted behind the transmitter reflector and well inside the focal point of the receiver reflector, so reducing the total axial depth of the antenna.

Another object of this invention is to provide a coaxial antenna in which a shallow, compact receiver reflector may be used, sincethe focal point thereof may to serve as a mounting for the receiver feed unit which i is thus close to the antenna mounting and the detector equipment and concentrates much of the antenna mass adjacent the rotatable servo mechanism.

Another object of this invention is to provide a coaxial antenna in which the transmitter and'receiver elements are mounted together in a rigid unitary structure, having no moving parts to cause undesirable vibration.

Finally, it is an object to provide a coaxial antenna of the aforementioned character which is simple and convenient to. buildand install and which will give generally efficientand durable service.

With these and other objects definitely in view, this invention consists in the novel construction, combination and arrangement of elements and portions, as will be hereinafter fully described in the specification, particularly 'pointed'out in the claims, and illustrated in the drawing which forms a material part of this disvclosure, and in which:

Figure 1 is a front elevation view of the antenna; and

Figure 2 is a sectional view taken on the line 2-2 of Figure 1.

Similar characters of reference indicate similar or identical elements and portions throughout the specification and throughout the views of the drawing. 7

Referring now to the drawing in detail, one embodiment of this invention is represented as an antenna comprising a large dish-like first reflector hereinafter considered to be a receiver reflector 10, substantially parabolic in cross section, and a smaller parabolic dish type second reflector hereinafter considered to be a transmitter reflector 12. Fixed concentrically to the inner surface of the receiver reflector It) is a forwardly extending support cylinder 14-, of plastic or other non-metallic material, and the transmitter reflector 12 is fixed adjacent the forward end of the support cylinder, so that a small Patented June 21, 196 0 ice flector to serve as an isolating shroud 16. The central portion of the receiver reflector 10 within the support cylinder 14 is shielded by the transmitter reflector 10 and is thus ineflective as a reflector element, so this portion is made flat as indicated at 18, this flattening conserving space, minimizing axial length and providing a useful mounting surface for attachment to the antenna serve mechanism, indicated in broken lineat 20.

Fixed inside the support cylinder 14 immediately behind the transmitter reflector 12 is a third reflector, represented as a flat, disc-like plane reflector 22, which is preferably of plastic or the like and has a metallic reflective layer 24 on the surface facing the fla-t portion '18. The receiver feed can be considered as comprised of the third reflector 22 and the feed unit proper identified later in this specification by the numeral 38, and this receiver feed can, therefore, be properly considered to 'be a front feed. The receiver reflector 10 and transmitter reflector 12 are also preferably of plastic material, either molded of glass fiber reinforced plastic or of sandwich or honeycomb structure. The parabolic annular portion 26 of the receiver reflector 10 'has a reflective layer 28 on its concave surface and the transmitter reflector 12 is similarly provided with a reflective layer 30,- which extends over the isolating shroud 16 and over the end portion of the outer surface of the support cylinder .4, as indicated at 32. The metallic reflective material may be sprayed or otherwise deposited on the various surfaces as required. With the major structural components made of plastic material, the antenna may be bonded together into a rigid unitary structure. However, metallic material may be used for the reflector elements if desired.

The transmitter reflector 12 is illuminated by a conventional front feed 34 mounted at the end of a waveguide 36, which is secured to-the edges of both re; flectors 10 and 12, said waveguide being suitably connected to a microwave transmitter. The receiver feed unit 38 is mounted on the flat portion 18 inside the support cylinder 14 and is thus conveniently near the servo mechanism 20 and the receiver unit which is usually in that vicinity. i

The transmitter reflector 12 emits a narrow microwave beam in the normal manner and the echo signals are picked up by the receiver reflector 12. These received signals are reflected through the wall of the support cylinder 14, which is merely a support for the transmitter reflector 12 and does not obstruct the microwave energy. However, the wall thickness of the support cylinder 14 is determined according to the average angle of incidence of the reflected waves from the receiver reflector 12, in order to minimize distortion. The received signalsare then reflected from the plane reflector 22 directly to the receiver feed 'unit 38, which is at an image point created by the plane reflector, or more specifically, by the re flector system comprised of the reflector 10 and the reflector 24. The large, shallow receiver reflector 12, as shown, has a focal point well forward of the dish and even ahead of the transmitter reflector 12. Positioning areceiver feed at the true focal point would result in a bulky assembly and would obstruct the antenna reflector surfaces. By utilizing a plane reflector 22 well within the focal length of the reflector and mounting the receiver feed 38, actually on the center of the receiver reflector 12, the microwave energy travels in a folded path of the correct focal length, while the axial dimension of the antenna is greatly reduced. The plane reflector 22 has been found to be very eflicient in operation, is probably not less critical than parabolic or other curved secondary reflectors and provides good edge illumination. Since the center portion of the receiver reflector cient.

an extremely compact assembly. The support cylinder 14 is nonobstructive to microwave energy and provides a very rigid support for the transmitter reflector and plane reflector 22, while shielding the receiver feed 38 from damage by foreign matter. The main mass of the anconcentrated at the rear near the gimbal point of the servo mechanism 20, resulting in better balance and greatly reduced loads on the mechanism.

The coaxial antenna system is particularly suitable for continuous wave seeker radar and the configuration allows the use of linear polarization- This eliminates the neceslsity for cross polarization or circular polarization complexities inherent in other types of antennas. The transmitter reflector 10 is effectively isolated from the receiver reflector 12 by the isolating shroud 16 which extends forwardly of said transmitter reflector. The unitary form of construction of the antenna with no moving parts,

greatly reduces its susceptibility to vibration and minimizes microphonics in the system.

Also, since no scanning or sweep devices are used within the antenna itself,

.both transmitter and receiver reflectors are totally reflecting simultaneously and consequently are fully efli- Search (broad sweep) as well as scanning is accomplished by swinging or rotation of the entire an tenna assembly in a conventional manner by means of 'the servo mechanism 20 and by the use of a special type of receiver feed, which is the subject of a copending application.

The operation of this invention will be clearly comprehended from a consideration of the foregoing description of the mechanical details thereof, taken in connection with the drawing and the above recited objects. It will be obvious that all said objects are amply achieved by this invention.

It is understood that minor variation from the form of the invention disclosed herein may be made without departure from the spirit and scope of the invention, and that the specification and drawing are to be considered as merely illustrative rather than limiting.

I claim:

1. Av coaxial antenna, comprising: a substantially parabolic receiver reflector having a generally flat central portion; a receiver feed operatively mounted centrally on said flat portion; a smaller, substantially parabolic transmitter antenna operatively fixedly mounted concentric with and axially spaced from said receiver reflector; a flat plane reflector fixedly mounted between said transmitter reflector and receiver reflector and having its reflective surface directed toward said receiver feed, the combined distance from said receiver reflector to said plane reflector and from the plan'e reflector to said receiver feed being substantially equal to the effective focal length of the receiver reflector,

2. A coaxial antenna, comprising: a substantially parabolic receiver reflector having a generally flat central portion; a receiver feed operatively mounted centrally on said flat portion; a smaller, substantially parabolic transmitter antenna operatively fixedly mounted concentric with and axially spaced from said receiver reflector; a flat plane reflector fixedly mounted between said transmitter reflector and receiver reflector and having its reflective surtenna, including the normally bulky receiver feed, is i face directed toward said receiver feed, the combined distance from said receiver reflector to said plane reflector and from the plane reflector to said receiver feed being substantially equal to the effective focal length of the receiver reflector; and an electromagnetic isolating shroud extending axially from the periphery of said transmitter reflector away from said receiver reflector.

3. A coaxial antenna, comprising: a substantially parabolic receiver reflector having a generally flat central portion; a receiver feed operatively mounted centrally on said flat portion; a generally cylindrical combined shield and support element fixed. at one end.to said receiver reflector and enclosing said flat portion, said support element being substantially non-obstructive to microwave energy; a smaller, substantially parabolic transmitter reflector fixed to said support element adjacent the other end thereof and concentric with said receiver reflector; an electromagnetic isolating shroud coextensive with said support element and extending axially beyond the periphery of said transmitter reflector; a flat plane reflector fixedly mounted between said transmitter reflector and receiver reflector and having its reflective surface directed toward said receiver feed, the combined distance from said receiver reflector to said plane reflector and from the plane reflector to said receiver feed being substantially equal to the effective focal length of the receiver reflector.

4. A coaxial antenna capable of continuous wave transmission and reception comprising: a first reflector and a second reflector coaxial therewith; each of said reflectors having a front feed; said second reflector being disposed radially outwardly and slightly rearwardly of said first reflector; said front feed of the second reflector including a third mirror on the back of said first reflector and a feed unit mounted substantially on the surface of said second reflector at the effective image focal point of said second reflector with respect to wave energy directed parallel to the axis of the antenna and reflected by said second reflector; the true image point of said second reflector being considerably forwardly beyond said first reflector.

5. A coaxial antenna according to claim 4 wherein said front and rear reflectors are parabolic.

6. A coaxial antenna according to claim 4 wherein said rear reflector is supported on a central base corresponding generally with said front reflector in area, said base being substantially flat to minimize axial length of the complete antenna and having support means for the front reflector; and said base constituting support means for said feed unit of said rear reflector.

7. An antenna according to claim 6 wherein said support means is a cylinder of material characterized by exhibiting minimal interference with microwave energy,

7 said support means being a shield for said feed unit against foreign matter, said front reflector and said third reflector being peripherally mounted on said cylinder.

8. A coaxial antenna according to claim 4 wherein said third reflector is flat;

References Citedin the file of this patent UNITED STATES PATENTS Cochrane Feb. 28, 1956 

